1. Field of Invention
The present invention relates to liquid crystal devices used in printers, and in particular to a liquid crystal print bar using thin film transistors and transient response.
1. Description of the Prior Art
Print bars are rapidly becoming a necessary component in printers and multifunction electronic reprographic systems. The optical print bar offers the convenience of making the office copier serve as a computer printout device while still retaining its use as a copier. It is also possible to use the copier in facsimile transmissions. The use of liquid crystals in various configurations is well known. For example, U.S. Pat. No. 4,385,292 discloses a segmented liquid crystal display and a system for driving the display comprising thin film transistors coupled to a plurality of segmented display electrodes. U.S. Pat. No. 4,386,352 discloses a matrix liquid crystal display comprising a thin film transistor array and U.S. Pat. No. 4,403,217 discloses a multiplex liquid crystal display, the display comprising a thin film dielectric layer. U.S. Pat. No. 3,936,172 teaches the use of a liquid crystal material at the platen station in a reproduction machine to mask predetermined regions of an original document.
It is also known to use light emitting diodes (LED's) in alpha numeric devices. For example, IBM Technical Disclosure Bulletin, Vol. 13, No. 12, May 1971, teaches the selective excitation of light emitting diodes by a computer to produce image patterns. In addition, U.S. Pat. No. 3,438,057 discloses the parallel excitation of selected ones of a plurality of solid state light emitters mounted in a linear array in registration with a recording station to record multi-channel data on a photographic film. U.S. Pat. No. 3,850,517 uses a row of LED's and the motion of a photoconductor for vertical scan. Other uses of an array of solid state light emitters are to illuminate a document as disclosed in U.S. Pat. No. 3,967,893.
It is also known to provide a printhead with a ferro-electric liquid crystal light shutter array. The head is constructed from a florescent lamp, a light shutter array using liquid crystals and a focusing lens as disclosed in SID'85 Digest, pages 373-376, dated May 1985. In addition, SID '86 Digest, pages 263-265 discloses a liquid crystal shutter for a printer based on a dual frequency addressed G. H Mode and the publication, Technology, dated May 1986, generally discloses the use of liquid crystal shutters for light control and non-impact printers.
A difficulty with the prior art print bars is generally the complexity and cost of the print bar as well as the difficulty in obtaining high resolution and speed when used as a printer.
It is an object of the present invention, therefore, to provide a new and improved print bar using liquid crystal shutters. Another object of the present invention is to provide a liquid crystal array print bar incorporating thin film transistor drivers for high multiplexability, speed and reduced cost. It is still another object of the present invention to drive each liquid crystal picture element with a separate thin film transistor and then multiplex the input signals to the thin film transistors. It is still another object of the present invention to use the transient electro-optic effect within the liquid crystal shutters to increase the printing speed at which the liquid crystal print bar can operate. It is still another object of the present invention to simplify the system and reduce cost by the use of two voltage level drivers to avoid the complication of using three or more voltage level drivers. Further objects and advantages of the present invention will become apparent as the following description proceeds and the features characterizing the invention will be pointed out with particularity in the claims next to a forming a part of this specification.